1. Field of the Invention
The present invention relates to a screen-printing method and a screen-printing apparatus and particularly to the art of facilitating the separation from a screen of a print material printed on a substrate.
2. Discussion of Related Art
A screen printing is carried out by filling, in a state in which a screen is contacted with a substrate such as a printed wiring board, through-holes of the screen with a print material such as creamed solder, and thereby applying the print material to the substrate. After the printing operation, the screen is separated from the substrate, so that the print material is separated from the screen and is left on the substrate. Thus, the print material is printed on the substrate.
However, there is a problem that when the screen and the substrate are separated from each other after the printing operation, a certain amount of the print material remains adhered to respective inner surfaces of the through-holes of the screen, so that an amount of the printing material left on the substrate may be short or a print pattern formed on the substrate may have a defect.
The present invention provides a screen-printing method and a screen-printing apparatus which have one or more of the following technical features that are described below in respective paragraphs given parenthesized sequential numbers (1) to (19). Any technical feature that includes another technical feature shall do so by referring, at the beginning, to the parenthesized sequential number given to the latter feature. However, the following technical features and the appropriate combinations thereof are just examples to which the present invention is by no means limited. In addition, in the case where one technical feature recites a plurality of items, it is not essentially required that all of those items be simultaneously employed. That is, it is possible to select and employ only a portion (one, two, . . . , but not all) of those items.
(1) According to a first feature of the present invention, there is provided a screen-printing method, comprising the steps of filling, in a state in which a screen having a plurality of through-holes is contacted with a print surface of a substrate, the through-holes with a print material, and thereby applying the print material to the print surface of the substrate, moving, after the filling step, at least one of the screen and the substrate relative to the other of the screen and the substrate, in at least one cycle, on a plane parallel to the print surface of the substrate, in at least one direction parallel to at least one straight line, and separating, after the moving step, the screen and the substrate from each other.
When one of the screen and the substrate is moved relative to the other of the screen and the substrate, in a direction parallel to the print surface of the substrate, the print material, i.e., a print pattern formed by the print material filled in a through-hole and applied to the substrate is moved relative to the through-hole, so that a clearance is produced between a portion of the entire periphery of the print pattern and an inner surface of the through-hole. Therefore, when the screen and the substrate are separated from each other after the relative movement thereof, the print pattern is easily separated from the through-hole, such that no print material, or only a small amount of print material, if any, is left on the side of the screen. Thus, the present screen-printing method is free of the problems that the print material is short or the print pattern has a defect.
The number of cycles in which the screen and the substrate are moved relative to each other may be determined depending on, for example, the degree of difficulty of printing or the degree of difficulty of separation of the print pattern from the through-holes. For example, in the case where the screen has a constant thickness and the through-holes have a substantially identical cross-sectional shape, the degree of difficulty of separation of the print pattern increases as the cross-sectional area of the through-holes decreases. Therefore, the number of cycles increases. In this case, a circular through-hole and a square through-hole are defined as having a substantially identical cross-sectional shape. In addition, in the case where the screen has a constant thickness, and the through-holes have different cross-sectional shapes but have a substantially identical cross-sectional area, the degree of difficulty of separation of the print pattern increases and the number of cycles increases as a value (i.e., a ratio) obtained by dividing a maximum inner dimension of each through-hole by a minimum inner dimension thereof increases. For example, in the case where the through-holes have a rectangular, shape, the above ratio increases as the length of the short sides of the rectangular through-holes decreases. Moreover, as a value (i.e., a ratio) obtained by dividing the depth of each through-hole by the cross-sectional area thereof increases, the degree of difficulty of separation of the print pattern increases and the number of cycles increases. If the number of cycles is determined in this way, then it is assured that respective clearances are produced between the print pattern and the respective inner surfaces of the through-holes, according to the shape, dimensions, and/or height (i.e., depth) of each through-hole, and that the screen and the substrate are moved relative to each other with a small amount of waste.
(2) According to a second feature of the present invention that includes the first feature (1), the one cycle comprises a first forward movement of the at least one of the screen and the substrate by a distance xcex1, a backward movement of the at least one of the screen and the substrate by twice the distance xcex1, and a second forward movement of the at least one of the screen and the substrate by the distance xcex1, in an order of description.
According to this feature, in a state after the second forward movement following the backward movement, two clearances each having a dimension substantially equal to the distance xcex1, are produced between opposite end portions of each print pattern as seen in the direction of relative movement of the screen and the substrate, and opposite end portions of an inner surface of a corresponding through-hole as seen in the same direction. Although the size of each print pattern is decreased since the clearances are produced, the center of the each print pattern is not moved in the above-indicated direction and accordingly the each print pattern is formed at a correct position. Thus, for example, in the case where the substrate is a printed wiring board and the print material is creamed solder, when an electric component having a lead wire is mounted on the wiring board, the lead wire is placed at the center of a print pattern in a widthwise direction thereof, so that an electric circuit (i.e., an electric-conductor pattern) is produced with reliability.
(3) According to a third feature of the present invention that includes the second feature (2), the distance a is not smaller than one hundredth of an inner dimension of one of the through-holes in the one direction and not greater than one fifth of the inner dimension.
In the case where all the through-holes are elongate in the one direction parallel to the one straight line, the relative movement of the screen and the substrate in one or more cycles in only the one direction suffices in many cases. More specifically described, providing that an elongate through-hole is divided into unit-length portions in a lengthwise direction thereof lengthwise opposite end unit-length portions of the through-hole are more contacted with a print pattern than intermediate unit-length portions of the through-hole and, when the screen is separated from the substrate, the print material has a difficulty to separate from the inner surface of the through-hole and accordingly a certain amount of the material is left on the side of the screen. In contrast, when the screen and the substrate are moved relative to each other in a direction parallel to the lengthwise direction of a through-hole, the through-hole and a print pattern are moved relative to each other in the direction parallel to the lengthwise direction of the through-hole, so that respective clearances are produced between the lengthwise opposite end portions of the print pattern and the lengthwise opposite end portions of the inner surface of the through-hole, where the print material has a difficulty to separate from the through-hole. Thus, the print material can be easily separated from the through-hole. Since the clearances are present in the lengthwise opposite end portions of the through-hole, the print pattern is contacted with the lengthwise opposite end portions of a pair of long sides of the through-hole, by the same area as that by which the print pattern is contacted with the other, intermediate portions of the long sides of the through-hole.
If the distance xcex1 is too small, the advantage is too low; and if the distance xcex1 is too great, the print pattern as the shape of the print material printed is deformed too much. Hence, it is preferred that a lower limit of the distance a be one two-hundred-and-fiftieth, one hundredth, two hundredths, or three hundredths of an inner dimension of one through-hole and that an upper limit of the distance xcex1 be one fifth, fifteen hundredths, ten hundredths, or seven hundredths of the inner dimension. The dimension of each of the clearances produced between a print pattern and a through-hole may be generally proportional to the inner dimension of the through-hole in the direction of movement of the through-hole. However, even if the inner dimension of the through-hole may be considerably small, the dimension of each clearance needs to be greater than a certain lower limit; and even if the inner dimension may be considerably great, the dimension of each clearance need not to be greater than a certain upper limit. Thus, it is natural that the dimension of each clearance should have an upper and a lower limit. Upper and lower limits of a ratio of the distance a to the inner dimension of a through-hole decreases as the inner dimension increases, and vice versa. This ratio changes depending upon the inner dimension of the through-hole. More specifically described, the upper and lower limits of the ratio are selected from a range of greater ratios in the case where the inner dimension is smaller; and the upper and lower limits of the ratio are selected from a range of smaller ratios in the case where the inner dimension is greater. However, generally, it is preferred that the distance xcex1 have the above-indicated upper and lower limits.
(4) According to a third feature of the present invention that includes any one of the first to third features (1) to (3), the plurality of through-holes comprise at least one first elongate hole which is elongate in a first direction parallel to a first straight line parallel to the print surface, and at least one second elongate hole which is elongate in a second direction parallel to a second straight line parallel to the print surface and perpendicular to the first straight line, and the moving step comprises moving, in each of the first and second directions, the at least one of the screen and the substrate relative to the other of the screen and the substrate, in the at least one cycle.
The advantage is obtained to some extent by moving, in one or more cycles, one of the screen and the substrate relative to the other of the screen and the substrate in only one direction parallel to one of the first and second straight lines. However, it is preferred to move, as described above, one of the screen and the substrate relative to the other of the screen and the substrate, because the first elongate hole is moved relative to a corresponding print pattern in the first direction, i.e., in the lengthwise direction thereof, and the second elongate hole is moved relative to a corresponding print pattern in the second direction, i.e., in the lengthwise direction thereof. The relative movement of the screen and the substrate in the first direction parallel to the first straight line, and the relative movement of the screen and the substrate in the second direction parallel to the second straight line may be effected at fully different timings, or at least partly concurrently. The latter manner may be carried out by moving, in at least a portion of the duration of the moving step, one of the screen and the substrate relative to the other of the screen and the substrate so as to describe a circular locus. In the latter manner, if the screen is moved relative to the substrate to describe a circular locus and additionally an arbitrary point on the screen is moved, during one cycle, relative to the substrate in a substantially diametrical direction of the circle, in the order recited in the second feature (2), a uniform clearance is produced around the entire periphery of a print pattern.
(5) According to a fifth feature of the present invention that includes any one of the first to third features (1) to (3), the plurality of through-holes comprise at least one first elongate hole which is elongate in a first direction parallel to a first straight line parallel to the print surface, and at least one second elongate hole which is elongate in a second direction parallel to a second straight line parallel to the print surface and perpendicular to the first straight line, and the moving step comprises moving, in a third direction parallel to a third straight line inclined with respect to each of the first and second straight lines and parallel to the print surface, the at least one of the screen and the substrate relative to the other of the screen and the substrate, in the at least one cycle.
According to this feature, the third straight line has a first component parallel to the first straight line and a second component parallel to the second straight line, and each of the first and second elongate holes and a corresponding one of two sorts of print patterns formed by filling those elongate holes with the print material are moved relative to each other, concurrently in both the lengthwise direction of the each elongate hole and a direction perpendicular to the lengthwise direction. Therefore, a clearance is efficiently produced around the entire periphery of each print pattern.
(6) According to a sixth feature of the present invention that includes any one of the first to third feature (1) to (3), the moving step comprises moving the at least one of the screen and the substrate relative to the other of the screen and the substrate, in the at least one cycle, on the plane parallel to the print surface of the substrate, in each of an X-axis direction and a Y-axis direction parallel to an X axis and a Y axis, respectively, which are perpendicular to each other on the plane.
The explanations provided for the fourth feature (4) are true with the present feature.
(7) According to a seventh feature of the present invention that includes the sixth feature (6), the moving step comprises moving the at least one of the screen and the substrate relative to the other of the screen and the substrate, to describe such a relative-movement locus that an arbitrary point on the screen moves, relative to the substrate, from an initial position of the arbitrary point to a point on a circle whose center is the initial position, and then moves on the circle.
According to this feature, a clearance is produced between almost the entire periphery of a print pattern and an inner surface of a through-hole, without moving the center of the print pattern out of position.
(8) According to an eighth feature of the present invention that includes the seventh feature (7), a portion of the relative-movement locus that corresponds to the movement of the arbitrary point from the initial position thereof to the point on the circle whose center is the initial position, comprises a first component in a circumferential direction of the circle and a second component in a radial direction of the circle.
Otherwise, the relative-movement locus may be one which consists of the second component in the radial direction of the circle. However, when an inner surface of a through-hole is moved relative to a print pattern to be separated from the print pattern, the shape of the print pattern is less damaged by moving the inner surface in a direction having a component parallel to the inner surface, than by moving the inner surface in a direction perpendicular to the inner surface.
(9) According to a ninth feature of the present invention that includes the eighth feature (8), the portion of the relative- movement locus that comprises the first and second components comprises a spiral curve whose diameter increases as the arbitrary point moves from the initial position thereof toward the circle.
Since the relative-movement locus includes the spiral curve, the shape of the print pattern is less damaged.
(10) According to a tenth feature of the present invention that includes any one of the first to third and sixth features (1) to (3) and (6), the moving step comprises moving the at least one of the screen and the substrate relative to the other of the screen and the substrate, to describe such a relative-movement locus that an arbitrary point on the screen moves, relative to the substrate, from an initial position of the arbitrary point, along a spiral curve whose diameter increases as the arbitrary point moves at least one time around the initial position.
(11) According to an eleventh feature of the present invention that includes any one of the first to third features (1) to (3), the moving step comprises moving the at least one of the screen and the substrate relative to the other of the screen and the substrate, to describe a circle whose center is an arbitrary point on the screen.
(12) According to a twelfth feature of the present invention that includes any one of the seventh to tenth features (7) to (10), the moving step comprises moving the at least one of the screen and the substrate relative to the other of the screen and the substrate, so that the arbitrary point on the screen moves back, relative to the substrate, to the initial position of the arbitrary point.
A relative-movement locus along which one of the screen and the substrate is moved back to its initial position relative to the other of the screen and the substrate may be the same as, or different from, that along which one of the screen and the substrate is moved from its initial position relative to the other of the screen and the substrate.
In each of the screen-printing methods recited in the seventh to ninth features (7) to (9), a portion of the relative-movement locus that corresponds to the movement of an arbitrary point on the screen back to its initial position may be a radius of the circle, or one having both a first component in a circumferential direction and a second component in a radial direction, such as a spiral curve whose radius gradually decreases.
In the screen-printing method recited in the tenth feature (10), a portion of the relative-movement locus that corresponds to the movement of one of the screen and the substrate back to its initial position relative to the other of the screen and the substrate may be a spiral curve, or a radius of a circle whose center is the initial position.
The relative movement of the screen and the substrate back to their initial relative position produces a clearance around the entire periphery of a print pattern, irrespective of which shape the print pattern may have. In particular, in each of the screen-printing methods recited in the seventh to ninth features (7) to (9), a uniform clearance is produced around the entire periphery of a print pattern, without moving the center of the print pattern out of position.
(13) According to a thirteenth feature of the present invention that includes any one of the first to twelfth features (1) to (12), a period of the one cycle is not shorter than 0.001 second and not longer than 1 second.
The optimum period of one cycle of the relative movement of the screen and the substrate is determined mainly depending upon the viscosity of the print material. It is, however, preferred that the lower limit of the period be 0.001 second, 0.002 second, 0.005 second, or 0.01 second and the upper limit of the period be 1 second, 0.4 second, or 0.1 second.
(14) According to a fourteenth feature of the present invention that includes any one of the first to thirteenth features (1) to (13), the moving step comprises moving the at least one of the screen and the substrate relative to the other of the screen and the substrate, and simultaneously vibrating at least one of the screen and the substrate at a period shorter than a period of the one cycle.
This feature promotes producing a clearance between a print pattern and an inner surface of a through-hole.
(15) According to a fifteenth feature of the present invention that includes the fourteenth feature (14), the vibrating step comprises vibrating the at least one of the screen and the substrate at a supersonic frequency.
(16) According to a sixteenth feature of the present invention that includes any one of the first to fifteenth features (1) to (15), the moving step comprises moving, with an actuator which is for correcting at least one positional error between the screen and the substrate in the at least one direction parallel to the print surface of the substrate, the at least one of the screen and the substrate relative to the other of the screen and the substrate.
Since the positional error between the screen and the substrate is corrected, the print material is accurately printed on a print position on the substrate. The actuator for correcting a small positional error between the screen and the substrate can be used to move, by a small distance, one of the screen and the substrate relative to the other of the screen and the substrate. According to this feature, the actuator can be used for not only correcting the position and but producing the clearances. Thus, the clearances can be produced with ease and at low cost.
(17) According to a seventeenth feature of the present invention that includes the sixteenth feature (16), the actuator comprises an electric motor which is controllable with respect to an angle of rotation thereof.
The electric motor according to this feature may be a servomotor or a stepper motor. According to this feature, the distance or speed of relative movement of the screen and the substrate can be easily controlled, and a clearance having a desired dimension can be easily produced.
(18) According to an eighteenth feature of the present invention, there is provided a screen printing apparatus, comprising a screen holding device which holds a screen having a plurality of through-holes; a substrate holding device which holds a substrate; a first moving device which moves at least one of the screen and the substrate toward, and away from, the other of the screen and the substrate, so that the screen and the substrate are contacted with, and separated from, each other; a filling device which fills, in a state in which the screen is contacted with a print surface of the substrate, the through-holes with a print material and thereby prints the print material on the print surface of the substrate; a second moving device which moves, in the state in which the screen is contacted with the substrate, at least one of the screen holding device and the substrate holding device relative to the other of the screen holding device and the substrate holding device, on a plane parallel to the print surface of the substrate, in at least one direction parallel to at least one straight line; and a control device which controls the first moving device, the filling device, and the second moving device, the control device comprising a clearance-produce control portion which controls the second moving device to move at least one of the screen and the substrate relative to the other of the screen and the substrate, in at least one cycle, and thereby produce respective clearances between respective inner surfaces of the through-holes of the screen and the print material printed on the print surface of the substrate.
The present screen-printing apparatus may employ one or more of the above-described second to seventeenth features (2) to (17).
The present screen-printing apparatus can enjoy one or more of the advantages of the screen-printing method according to the first feature (1).
(19) According to a nineteenth feature of the present invention that includes the eighteenth feature (18), the screen-printing apparatus further comprises a positional-error detecting device which detects at least one positional error between the screen and the substrate in the at least one direction parallel to the print surface of the substrate, and the control device further comprises a position-correct control portion which controls, based on the positional error detected by the positional-error detecting device, the second moving device to reduce the detected positional error.
It is preferred that the second moving device be one which can be controlled by the position-correct control portion to reduce the positional error between the screen and the substrate, to substantially zero. However, this is not essentially required. That is, the second moving device may be one which can at least partly reduce the positional error.
The present screen-printing apparatus can enjoy the advantages of the screen-printing method according to the sixteenth feature (16).